The present invention relates to fuel cells, and more particularly to transient temperature control of the fuel processor during startup.
Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have also been proposed for use in electrical vehicle power plants to replace internal combustion engines. The acceptance of fuel cells by drivers will be governed, in part, by their experience with vehicles powered by the internal combustion engine. Consumers have grown accustomed to the relatively quick starting times of internal combustion engines. One challenge facing fuel cell designers is to provide a similar relatively quick starting time for fuel cells. This is made difficult by the relatively high operating temperatures of some of the components of fuel cells such as the primary reactor of the fuel processor.
A solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) or reformate is supplied to the anode and air or oxygen (O2) is supplied to the cathode. In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (exe2x88x92). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across electrode plates. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (exe2x88x92) are taken up to form water (H2O).
A fuel cell can be operated using pure hydrogen. When hydrogen is used, the fuel cell has zero emissions since the product of the fuel cell is water. Fuel cells that use pure hydrogen as fuel, however, have significant commercial disadvantages. Currently, there is no commercial distribution system for hydrogen. Hydrogen is also difficult to store and distribute, so it would be much more convenient if fuel cells use fuels that are more readily available. As such, a fuel processor is typically used to convert fuels other than pure hydrogen into hydrogen. For example, the fuel processor can reform hydrocarbon or alcohol fuels such as natural gas, propane and methanol into hydrogen, which is then supplied to the fuel cell. The main function of the fuel processor in a fuel cell system is to provide a continuous stream of hydrogen to the fuel cell stack which converts the chemical energy in the hydrogen fuel to electric power. After start-up, the fuel cell stack demands a certain flowrate of hydrogen from the fuel processor to meet the vehicle""s demand for power.
The reformation process is typically optimized at high temperatures (about 600-800xc2x0 C.) relative to ambient tempertures. Thus, it is necessary that the catalytic reactors in the system (i.e., primary reformer and any associated secondary reformers such as water gas shift reactors or preferential oxidation reactors) are heated to a temperature sufficient to fully optimize there reformation process. During operation, the heat generated at various stages of the reforming process can be distributed and thereby controlled for optimum operation. During start-up, however, the reforming process must be modified so that operation of the various components are staged. This approach can lead to undesirable lag time for bring the system on-line. Alternately, external electrical heat sources (i.e., heaters) may be employed to bring the components to proper operating temperatures. This approach requires an external source of electricity such as a battery.
Accordingly, there exists a need in the relevent art to provide a system that is capable of heating the reforming system to operating temperatures upon start-up. Furthermore, there exists a need to provide a system that maximizes heat input into the reforming system while reducing large transient temperature variations during start-up.
A fuel processor for a fuel cell system according to the invention includes a thermal start device, a mixing region and a temperature control element. The temperature control element is located between the thermal start device and the mixing region. The temperature control element is a passive device that reduces transient temperature variation during thermal startup.
In other features of the invention, a flame arrestor is connected to the mixing region. A primary reactor is connected to the flame arrestor.
According to still other features of the invention, the transient temperature control element has a body defining an inlet and an outlet. A plurality of bores form passages between the inlet and the outlet. In a preferred embodiment, the temperature control element has a cylindrical shape.
According to other features of the invention, a reduction in transient temperature variation that is provided by the temperature control element is related to a number of bores formed in the end plate, a length of the temperature control element, and an area of the passages.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.